I will mention the fact that it’s not clear what the page is about until you scroll down to the bottom (not to me anyway). The form is pretty much a “call to action” but the reader does not yet know what he/she is signing up for. You might try making the very first section up top more informational and attention-grabbing. You can place a “call to action” link in the section which takes the reader to the form section. If that makes sense.
Have you considered using CSS Grids for this site? I’m not sure if it would make your video frame more responsive but it’s possible. The video iframe would be contained in it’s own grid section and you might be able to set values for the iframe that respond to the grid container around it. Also the fact that this page layout is already a grid made me think that it might be quicker and easier to format than flex layout.